1. Field of the Invention
The present invention relates generally to a mine roof support and, more particularly, to a pumpable mine roof support which is inexpensive to transport, can be erected on-site and has a reinforced structure with residual yield strength.
2. Description of Related Art
Various roof support devices in the prior art have been designed and used to provide support to a mine roof. Deep mining results in removal of material from the interior of a mine, thereby leaving unsupported voids of various sizes within the mine. These unsupported voids are conducive to mine roof buckling and/or collapse. Thus, it has been desirable to provide support to mine roofs to prevent, delay, or control collapse thereof.
Longwall shield systems are often used to prop up a roof during mining/tunneling. Some current systems use hydraulic rams which can adjust the height of the loading point against the roof. This type of system can adjust according to a certain amount of pressure from above with a desired yield of hydraulic fluid. However, when the load becomes too heavy, the hydraulic ram loading point can puncture through the roof. These shields are typically positioned on the active coal mining face. Supplemental roof supports are typically located in the tailgate roadway between longwall panels. The reason for the supplemental roof supports is to keep the tailgate open. The side weight from the last mined panel and the forward abutment weight from the active longwall panel can crush the tailgate closed, which blocks the airway needed to carry away dust and gas.
U.S. Pat. No. 5,308,196 to Frederick discloses another type of prior art mine roof support. Specifically, the Frederick patent discloses a confined core mine roof support including a container and compressible filler placed within the container. Installation of the roof support requires use of wood footing material at the base and top of the roof. The footing material is used to fill any remaining voids between the top of the roof support and the mine roof.
The use of wood as the footing material has numerous disadvantages. For example, the use of wood as the footing material causes the footing material to be susceptible to rot or other damage, which, over time, will lessen the structural integrity thereof and overall safety of the mine roof support installation. Additionally, since the remaining void between the top of the roof support and the mine roof is variable, correspondingly-sized footing material is required for each and every installation of the prior art mine roof support. Collecting and appropriately sizing the footing material for use in each installation is time consuming. Furthermore, timber for use as the footing material is relatively expensive. Also, due to the fact that the wood footing material may not necessarily be engineered, there is an inherent uncertainty and risk associated with the load capacity attributed to each piece of wood footing material. Specifically, each piece of wood footing material may absorb a varying amount of compressive force. Thus, there is a need to perform scheduled monitoring of each roof support installation to ensure that the wood footing material has completely settled and remains in compressed contact with the mine roof. Thereafter, additional wood footing material (e.g., wedges) may be necessary.
Another type of mine roof support comprises a Can support. This type of support is known for its stability and high yield performance and can provide support capacities ranging from 60 to 200 tons per support unit. The Can support also performs well in both high mining heights and high deformation environments that include 2-3 feet of floor heave that produces large lateral displacement of the base of the Can relative to the roof contact. FIG. 1 shows an example of a Can support 10 which is laterally displaced with respect to a roof 12 and floor 14 of the mine. The Can support has several disadvantages. One disadvantage is that it has to be topped off to establish roof contact and transportation difficulties due to its bulky size, particularly in lower seam operations. Normally, the support is topped off with wood crib timbers; however, this softer timber material can significantly degrade the stiffness of the support and stability if not properly installed. Another disadvantage of the Can support is that, once a certain load threshold is exceeded, the Can support can puncture through the roof. Yet another disadvantage of the Can support is that, after a certain degree of lateral displacement is exceeded, the one-piece Can support can tip over.
Yet another type of mine support was first developed in the United States by MICON, the Assignee of the present invention, in the early 1980's as an emergency action to help support a deteriorated longwall tailgate return airway that was collapsing. The supports included slip forms that were filled with pumpable material from the floor up to the roof in multiple lifts. The shapes of the bags were square, rectangular, and eventually round. Another type of pumpable mine support has been developed by Heitech (part of Heintzmann Corporation) and shown in FIG. 2, wherein pumpable load-bearing material is pumped into a fabric bag 20 hung from a mine roof 22. The pumpable load-bearing material is transported through tubular members 26 within the mine and through an opening 28 in the bag 20. While this fabric bag 20 provides a structure to form the support and provides confinement to the load-bearing material, this pumpable support sheds considerable load during post peak support. This is because the fabric bag 20 does not have the rigidity of the steel Can support and cannot provide sufficient confinement to prevent this load shedding. A residual load of up to 200 tons can be maintained through several inches, however, the pumpable bag arrangement will not have the residual strength of a Can support. Also, once a certain load threshold is exceeded, the pumpable material can bulge against the bag.
U.S. Pat. No. 6,547,492 to Degville discloses an inflatable mine support comprising a steel tube, which is installed where it is desired, to provide support and a flexible bag located within the tube for receiving pumpable load-bearing material. This arrangement allows for adjustability of the support in that the bag conforms to irregularities in the roof and floor, eliminating the need for topping off with respect to the roof and floor surface. Also, this arrangement allows for inflation on site. However, this arrangement also has the disadvantage of the Can support in that it is contained within a steel tube and, thus, may not have the necessary residual yield capacity to avoid punching through the roof if subjected to a significant load.
U.S. Pat. No. 6,394,707 to Kennedy et al., entitled “Yieldable Mine Roof Support”, utilizes a telescoping, cylindrical, metal container into which a filler material is installed on site. The telescoping feature of this support assures direct contact of the support with the mine roof and floor, eliminating the need for wood cribbing. The shortcoming of this support is that the metal cylinder, which can provide half or more of the strength of the roof support, is not continuous from roof to floor. This support has an “oversized”, metal cylinder sliding upward from and over a smaller diameter, metal cylinder as the filler material is installed. Thus, the load capacity of this roof support is dependent on the strength of the filler material, which reaches its peak after an inch convergence, and achieves no strength from the vertical compression of the metal cylinders. Supplementary roof supports must sustain minimum loads from 100 to 200 tons for deflections up to 10 inches and beyond, and it would seem that this support could not perform as needed in an underground mine.
Cribs are required to provide a peak strength (e.g., 300,000 lbs.) above an initial amount of compression (e.g., 1 inch) and then a residual strength (e.g., 200,000 lbs.) over a subsequent, extended range of compression (e.g., 1-6 inches). The specification of peak strength, residual strength, and the compression range are functions of mine conditions, amount of over-burden, type of roof and floor material, and the like. A mining engineer specifies particular performance standards based upon a particular environment of use for which a crib must function as a load-carrying structure from its elastic to plastic range. The crib manufacturer/installer is then assigned the task of producing and/or installing a crib which meets these performance standards. Accordingly, there is a need in the art for a mine support which can be easily/quickly constructed and/or tailored in an economic manner to satisfy a myriad of mine conditions.